Synchronous Replication Versus Asynchronous Replication
TradeoffDistributed SystemsMidSeniorUpdated
Primary Postgres in us-east-1 replicating to a us-west-2 standby for DR. Writes are user-facing checkout transactions.
Short answer: Asynchronous replication (commit on primary, replicate after). Cross-region sync replication adds ~70ms to every commit, killing checkout latency.
The Two Options
A
Synchronous replication (commit waits for standby ack)
B
Asynchronous replication (commit on primary, replicate after)
Drill This: Your Pick First
The Better Pick
Option B
Asynchronous replication (commit on primary, replicate after)
Cross-region sync replication adds ~70ms to every commit, killing checkout latency. Async accepts a small RPO (recovery point objective) window of unreplicated writes for big latency wins.
When the Other One Wins
Synchronous replication (commit waits for standby ack)
Financial ledgers with zero-data-loss requirements pick sync (or quorum sync to 2 of 3 standbys). Spanner's TrueTime-based sync replication is the gold standard.